[SPARK-48889][SS] testStream to unload state stores before finishing

### What changes were proposed in this pull request?
In the end of each testStream() call, unload all state stores from the executor

### Why are the changes needed?
Currently, after a test, we don't unload state store or disable maintenance task. So after a test, the maintenance task can run and fail as the checkpoint directory is already deleted. This might cause an issue and fail the next test.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
See existing tests to pass

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes apache#47339 from siying/SPARK-48889.

Authored-by: Siying Dong <siying.dong@databricks.com>
Signed-off-by: Jungtaek Lim <kabhwan.opensource@gmail.com>

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamTest.scala

@@ -813,6 +813,13 @@ trait StreamTest extends QueryTest with SharedSparkSession with TimeLimits with
         case (key, None) => sparkSession.conf.unset(key)
       }
       sparkSession.streams.removeListener(listener)
+      // The state store is stopped here to unload all state stores and terminate all maintenance
+      // threads. It is necessary because the temp directory used by the checkpoint directory
+      // may be deleted soon after, and the maintenance thread may see unexpected error and
+      // cause unexpected behavior. Doing it after a test finishes might be too late because
+      // sometimes the checkpoint directory is under `withTempDir`, and in this case the temp
+      // directory is deleted before the test finishes.
+      StateStore.stop()
     }
   }